Yep, as mentioned above it's a radio comm phrase, but turned into slang (like everything else) by the military meaning something like "things are good"

When you answer a radio call you can report back how well you can understand the transmission. You use two ratings, both number from 1 to 5. The first number is the signal strength. The second number is the clarity (amount of static) aka (signal to noise ration).

So for a fair signal you might say 3x3, but if you can hear them loud and clear, then you say 5 by 5, or just 5by, or fiver or who knows what else since I was in.

GroundLimit Out. (not over and out, dunno where that came from ;)